1. Field of the Invention
The present invention relates to an optical amplification block having an optical amplification fiber or other optical amplifiers and to an optical amplification system using the optical amplification block.
2. Description of the Related Arts
In the optical communication, an optical amplification fiber consisting of an optical fiber of an optical waveguide structure doped with rare earth elements, such as an EDF (erbium doped fiber) (hereinafter referred to as xe2x80x9cdoped fiberxe2x80x9d), or an optical amplification system using a semiconductor optical amplifier consisting of a semiconductor laser is widely being used. The amplifying principle of an amplification fiber on such an optical amplification system is as described below. For the wavelength (for instance, 1.55 um band) of a signal light to be amplified, an excitation light of a specified wavelength (a wavelength which is 100 nm smaller than the wavelength of a signal light: 0.98 um or 1.4 um is selected) will be waveguided into a doped fiber. Under this condition, when a signal light is supplied, the dope element in a doped fiber (in the example shown above, Er: erbium) will be excited by the excitation light and an induced emission will be generated. As a result, signal lights will be amplified.
Here, on the optical amplification fiber of the structure as described above, if excitation is continued by an excitation light, when signal lights will not be supplied (when input is OFF), an optical surge of large power will be outputted, when input is resumed, and optical elements (parts) in the rear can be damaged or deteriorated.
Also, when the semiconductor optical amplifier is used, in order to obtain an output power which meets to an input light, a bias injection current is applied to the adjacent of a threshold of a semiconductor laser. And in order to keep the level of the output power constant, the bias injection current is controlled.
For the reason, if any signal light is not inputted (when the input is OFF), large bias injection current will be supplied. By this, when input is resumed, an optical surge of large power will be outputted.
Therefore, on conventional optical amplification systems, in order to judge the presence or absence of an optical input signal, part of the optical input signals is branched off by a coupler (branching device), and a level of the input light is monitored by a photodiode (PD) or other optical receiver element. When the optical input signal is down or OFF and the detected level of the optical signal becomes lower than the set value, excitation to the doped fiber is controlled to OFF (the supply of excitation light is stopped).
Also, when an auto control (ALC, AGC, etc.) to keep the output of the optical amplification fiber (or the semiconductor optical amplifier) constant, the coupler (branching device) is inserted to the output side of the doped fiber (or the semiconductor) to branch off part of the output. And, the branched part of the output lights is converted into electrical signals by the photodiode (PD) or other optical receiver element, while excitation lights (bias current) is controlled to bring the level to the set level.
Therefore, conventional optical amplification systems needed an optical receiver element to judge the presence or absence of an optical input signal, and an optical receiver element to control an optical output, and in addition, for these optical receiver elements, such an arrangement was needed so that couplers (branching devices) to branch and input an optical input signal and optical output signal would be used in pairs.
In view of the conventional configuration, the object of the present invention is to provide an optical amplification block that makes possible the shared use of an optical receiver element to judge the presence or absence of optical input signals and an optical receiver element to control an optical output, to thereby allow the configuration of one optical receiver element and of a driving circuit for driving the one optical receiver element to be reduced, and an optical amplification system using the amplification block.
In order to achieve the above object, according to a first aspect of the present invention there is provided an optical amplification block comprising an optical amplification media for making an optical amplification; a first branching device to input signal lights and branch the signal lights into first branched lights and second branched lights, in a predetermined proportion, and input the first branched lights to the optical amplification media; and a second branching device to input the output of the optical amplification media and the second branched lights, the second branching device branching the input lights into third branched lights as optical amplified output and fourth branched lights as monitor lights.
In order to achieve the above object, according to a second aspect of the present invention there is provided an optical amplification block comprising an optical amplification media for making an optical amplification; a first branching device to input signal lights and feedback branched lights and branch the input lights into first branched lights and second branched lights as monitor lights, in the specified proportions, the first branching device inputting the first branched lights to the optical amplification media; and a second branching device to input the output of the optical amplification media and branch the input lights into third branched lights as optical amplified outputs and fourth branched lights as feedback branched lights, in a predetermined proportion.
Preferably, the optical amplification block may further comprise removal means to remove ASE contained in optical signals amplified by the optical amplification media from the monitor lights.
The optical amplification media may be an optical amplification fiber to change the optical output level by excitation lights supplied, and the emission of the excitation lights supplied may be controlled depending on the level of the monitor lights.
The optical amplification media may be a semiconductor optical amplifier to change the optical output level by an input bias current, and the supply of the input bias current may be controlled depending on the level of the monitor lights.
In order to attain the above object, according to a third aspect of the present invention there is provided an optical amplification system comprising the amplification block; and an optical amplification control unit which inputs an optical signal level detected in conformity with the monitor lights in the amplification block, to compare the optical signal level with a first reference potential, the optical amplification control unit, when the optical signal level becomes lower than the first reference potential, stopping the emission of the excitation lights and comparing the optical signal level with a second reference signal to control the magnitude of the power of the excitation light depending on the magnitude of the difference from the second reference signal.
In order to attain the above object, according to a fourth aspect of the present invention there is provided an optical amplification system comprising the amplification block; and an optical amplification control unit which inputs the optical signal level detected in conformity with the monitor lights in the amplification block, to compare the optical signal level with a first reference potential to thereby control the magnitude of the excitation light depending on the magnitude of the difference from the first reference potential, the optical amplification control unit providing a control such that emission of the excitation light is stopped or permitted when the fall and the rise of the optical signal level to be detected in conformity with the monitor lights becomes lower than or higher than a predetermined potential.
The optical amplification control unit preferably includes a differentiation circuit to generate pulses for the fall and rise of the optical signal level to be detected.
The optical amplification control unit preferably includes a time constant circuit to compare the optical signal level with the first reference potential to delay the timing of detection at which the optical signal level becomes lower than the first reference potential.